Ms. Arroyo ordered security authorities to intensify their crackdown on what she called "lawless elements and terrorists."

Davao police Senior Superintendent Eric Javier says investigators have already identified a suspect in Wednesday's explosion. "There is one suspect who was described by one victim and he is supposed to be medium built, 5'7" and estimated to be aging about 36 or 37," he said.

He says initial tests show potassium nitrate was used in the bomb.

Several hours after Wednesday's blast, unidentified gunmen opened fire and threw grenades at three mosques in the city. Police would not say whether the attacks were in retaliation for the port bombing. No one was killed in the mosque attacks.

Until recently, Davao, a predominantly Christian metropolis in the Philippines's mostly Muslim southern islands, had been spared from decades of separatist conflict in the south. But on March 4, an explosion ripped through a waiting area at the Davao airport, killing 21 people. The leaders of the Muslim separatist group, the Moro Islamic Liberation Front or MILF, have been criminally charged with that attack.

No one has claimed responsibility for the latest incident. An MILF spokesman has denounced Wednesday's bombing.